1. Field of the Invention
The present invention relates to a process for the recovery of caprolactam from nylon-containing waste which has been converted to depolymerization products. After depolymerization, caprolactam is extracted from depolymerized products with use of alkyl phenolic type solvents.
2. Description of Related Art
Nylon containing waste, especially waste carpet simultaneously represents not only an environmental problem but also a potential source of value if useful materials can be recovered from it. For example, much waste carpet contains significant amounts of nylon, and in particular, nylon 6 or polycaprolactam. Processes for depolymerization of the waste nylon 6 to its valuable monomer, caprolactam, have been developed. Improvements in such processes, however, are still needed.
Nylon 6 depolymerization is usually effected in the presence of water. Although caprolactam is formed by depolymerization, the depolymerization reaction is an equilibrium reaction and at least some, and often significant, amounts of cyclic and linear caprolactam oligomers also form. Pure caprolactam, however, is ultimately desirable, but purification processes to separate the oligomers from the monomer can be cumbersome and expensive. Once separated from monomer, caprolactam oligomers can be recycled for further depolymerization. Therefore, despite the utility of known nylon 6 depolymerization and caprolactam purification processes, improved methods are still needed to make such processes economically attractive. Ideally, complete elimination of one or more purification steps is desirable.
Extraction is a method to separate caprolactam from complex mixtures resulting from nylon 6 waste depolymerization. For extraction to be effective, the solubilities of the different components must be carefully considered.
U.S. Pat. No. 5,359,062 discloses that nylon 6 can be depolymerized in the presence of water and alkali metal hydroxide to yield caprolactam- and oligomer-containing aqueous solutions. Following depolymerization, caprolactam can be separated by distillation or extraction. In order to have an economically attractive process it is essential to separate the oligomer from monomeric caprolactam and to recycle the oligomers.
The distillation route, however, suffers from the drawback that the distillation residue may not be readily recyclable because of tar formation. Other circumstances hindering the recycling of the residue include repolymerization and solidification of the oligomers. The extraction route suffers from, among other things, the need for large amounts of extraction agent. For example, more than twice the amount of extraction agent compared to the amount of caprolactam-containing aqueous solution may be needed. Moreover, although this patent disclosure mentions oligomer formation and separation, it does not disclose or suggest solutions to the particular problem of removing cyclic or linear oligomers from caprolactam. The extraction agents disclosed in U.S. Pat. No. 5,359,062 include benzene, toluene, and xylene, all of which are relatively low-boiling solvents compared to caprolactam. During extraction, these solvents extract not only caprolactam but also cyclic caprolactam oligomers. After extraction, these solvents can be removed from the caprolactam by distillation. An additional distillation step, however, is then needed to separate caprolactam from the cyclic oligomers. Hence, following the caprolactam extraction step, two distillations are still required to obtain the pure caprolactam.
U.S. Pat. No. 4,013,640 discloses an amide purification process which includes purification of caprolactam. This patent, however, also fails to address the problem of removing oligomers, and in particular, cyclic oligomers from caprolactam. Moreover, this patent publication also fails to disclose caprolactam purification processes which follow depolymerization of nylon 6. Rather, it discloses extraction of caprolactam from unspecified industrial waste waters. The disclosed impurities in these wastes are not impurities of similar chemical character as the amide to be purified. Extraction is effected with use of alkyl phenolic type solvents having boiling points higher than caprolactam. The extraction step is followed by vacuum distillation of the extract to obtain caprolactam. This distillation may be hampered by the presence of oligomers. U.S. Pat. No. 4,013,640, however, neither suggests nor discloses that alkyl phenolic extraction agents would be useful to separate caprolactam oligomers from caprolactam.